37*1 COMBINED ELECTROCHEMISTRY AND .../67531/metadc279396/...Fig.2.4c Ruthenium(II) complexes which are discussed in Chapter IV 22 Fig.2.5 Platinum(II) and palladium(II) complexes

3 7 * 1

/ / S / c /

y y a , V 7 f J

COMBINED ELECTROCHEMISTRY AND SPECTROSCOPY OF COMPLEXES

AND SUPRAMOLECULES CONTAINING BIPYRIDYL AND

OTHER AZABIPHENYL BUILDING BLOCKS

DISSERTATION

Presented to the Graduate Council of the

University of North Texas in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Lei Yang, B.S., M.S.

Denton, Texas

August, 1995

19

Yang, Lei, Combined Electrochemistry and Spectroscopy of Complexes and

Supramolecules Containing Bipyridyl and Other Azabiphenyl Building Blocks. Doctor

of philosophy (Chemistry), August, 1995, 184 pp., 13 tables, 87 illustrations,

references 196 titles.

A group of azabiphenyl complexes and supramolecules, and their reduced and

oxidized forms when possible, were characterized by cyclic voltammetry and

electronic absorption spectroscopy. The oxidized and reduced species, if sufficiently

stable, were further generated electrochemically inside a specially designed quartz cell

with optically transparent electrode, so that the spectra of the electrochemically

generated species could be taken in situ. Assignments were proposed for both parent

and product electronic spectra.

Species investigated included a range of Ru(II) and Pt(II) complexes, as well as

catenanes and their comparents.

Using the localized electronic model, the electrochemical reduction can be in

most cases assigned as azabiphenyl-based, and the oxidation as transition metal-based.

This is consistent with the fact that the azabiphenyl compounds have a low lying 7t*

orbital.

The electronic absorption spectra of the compounds under study are mainly

composed of n —> K* bands with, in some cases, charge transfer bands also.

ACKNOWLEDGMENTS

It is a privilege to express my sincere appreciation to professor Paul S.

Braterman, my major advisor. Without his constant instruction, guidance and

encouragement throughout this work, it would have been impossible to complete all

the work described here.

I would very much like to express my gratitude to Dr. Jae-In Song for his

advice, help and valuable comments. Special thanks also due to Dr. Frank M.

Wimmer of Universiti Brunei Darussalam, Bandar Seri Begawan, Brunei, for the

supply of platinum(II) complexes described in Chapter III, and also for his cooperation

on the work described in that chapter.

Thanks also due to Drs. G. Brent Young of Imperial College of Science,

Technology and Medicine, London, UK for the supply of the trimethylsilylmethyl

complexes and to J. Fraser Stoddart of The University, Sheffield, UK for the supply of

the catenanes and their precursors.

I also would like to acknowledge the financial support for this research from

Robert A. Welch foundation, and the University of North Texas Faculty Research

Fund.

m

TABLE OF CONTENTS

PAGE

LIST OF TABLES viii

LIST OF ILLUSTRATIONS x

CHAPTER

I. INTRODUCTION 1

1.1 Frontier Molecular Orbitals

1.2 Supramolecules

1.3 Azabiphenyl Systems

1.4 Symmetry Considerations

H. EXPERIMENTAL 12

2.1 Materials

2.2 Electrochemistry and Spectroelectrochemistry

2.3 The Compounds Under Examination

2.4 Quantum Chemistry Calculation

III. ELECTRO- AND SPECTROELECTROCHEMICAL STUDIES OF

PLATINUM(II) BIPYRIDINE COMPLEXES AND RELATED SPECIES 27

3.1 Electrochemistry of Bipyridines and Platinum(II) Complexes

3.1.1 Electrochemistry of Free bipyridines

3.1.1.1 Electrochemistry of 2,4'-bipyridine

IV

3.1.1.2 Electrochemistry of N'-Methyl-2,4'-Bipyridine

3.1.1.3 Electrochemistry of 4,4'-Diphenyl-2,2'-Bipyridine

3.1.2 Electrochemistry of Platinum(II) Complexes

3.1.2.1 Ligand-Based Reductions of Platinum(II) Complexes

[Pt(bipy)ClJ

[Pt(bipy)(4-NCpy)Cl]+

[Pt(ph2-bipy)Cy and [Pt(Me2-bipy)Cl2]

Pt(2,4'-bipyOct-H)Cl2

and [Pt(2,4'-bipyOct)Cl3]

[Pt(2,4'-bipyMe-H)(bipy)]2+

[Pt(2,4'-bipyMe-H)py2]2+

[Pt(Terpy)Cl]+

[Pt(DCMB)ClJ

3.1.2.2 Platinum(II)-Based Reductions

3.2 Spectroelectrochemistry

3.2.1 2,4'-Bipyridines

3.2.2 Spectroelectrochemistry of Square Planar Platinum(II) Complexes with

2,4'-Bipyridine as Ligand

[Pt(2,4-bipyOctyl)Cl3]

[Pt(2,4'-bipyOctyl-H)Cy

[Pt(bipy)Cy

[Pt(bipy)(4-CNpy)Cl]+

[PtDCMBClJ

IV. ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL STUDIES OF

SOME RUTHENIUM(II) AZABIPHENYL COMPLEXES

AND RELATED SPECIES 75

4.1 Electrochemical studies

4.1.1 Electrochemical studies of l,l'-Biisoquinoline

4.1.2 Electrochemical Studies of 2,2'-Bisquinoline

4.1.3 Electrochemistry of Ruthenium(II) Complex Ru(bipy)2(biiq)2+

4.1.4 Electrochemistry of Ruthenium(II) Complexes with Trialkyl Phosphite

and Trimethylsilylmethyl Ligands

4.2 Spectroelectrochemistry of [Ru(biiq)(bipy)2]2+ and Related Species

4.2.1 Spectroelectrochemistry of U'-Biisoquinoline

4.2.2 Spectroelectrochemistry of [Ru(biiq)(bipy)2]2+

4.2.3 Spectroelectrochemistry of Ruthenium(II) Complexes with Trialkyl

Phosphite and Trimethylsilylmethyl Ligands

V. ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL STUDIES OF

BIPYRIMIDINE PLATINUM(II) AND PALLADINUM(II) COMPLEXES 108

5.1 Electrochemistry of Mono- and Dinuclear Platinum(II) and Palladium(II)

Complexes

5.1.1 (bipym)Pt(CH2SiMe3)2

5.1.2 (bipym)Pd(CH2SiMe3)2

5.1.3 (Me3SiCH2)2Pd(bipym)Pt(CH2SiMe3)2

VI

5.1.4 (j-bipym(Pd(CH2SiMe3)2)2

5.2 Spectroelectrochemistry of Mono- and Dinuclear Platinum(II) and

Palladium(II) Complexes

VI. ELECTRO- AND SPECTROELECTROCHEMICAL STUDIES OF

PHENANTHROLINES 127

6.1 Electrochemistry of Phenanthrolines

6.2 Spectroelectrochemistry of Phenanthrolines

Vn. SPECTROELECTROCHEMICAL STUDIES OF SOME PARAQUAT

CATENANES AND THEIR PRECURSORS 145

7.1 Electrochemistry of Catenanes and Their Precursors

7.2 Spectroelectrochemistry of Catenanes and Their Precursors

VIII. COMPARISONS WITH MOP AC CALCULATION 164

8.1 Introduction

8.2 Performance of the Computations

8.3 Results of the Computations

IX. CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK 170

9.1 Some Concluding Points

9.2 Suggestions

BIBLIOGRAPHY 174

vu

LIST OF TABLES

PAGE

Table 3.1 Cyclic Voltammetry Data for Bipyridines 32

Table 3.2 Cyclic Voltammetry Data for Platinum(II) Complexes 37

Table 3.3. Spectroscopic Data and Proposed Assignments for Platinum Complexes and

Related Species in DMF 60

Table 4.1 Electrochemical data for Ru(Bipy)2(biiq)2+ and related species 77

Table 4.2 Electrochemical Data for Ruthenium(II)

Methylsilylmethyl Bipyridine Complexes 95

Table 4.3. Spectroscopic Data and Proposed Assignments for [Ru(l,r-biiq)(bipy)2]2+

and Related Species in DMF 96

Table 4.4. Spectroscopic Data and Proposed Assignments for Ruthenium(Il)

Methylsilylmethyl Bipyrimidine Complexes 99

Table 5.1 Cyclic Voltammetry Data for Trimethylsilylmethyl

Platinum(II) and Palladium(II) Complexes 109

Table 5.2 Spectroscopic Data and Proposed Assignments for Trimethylsilylmethyl

Platinum and Palladium Complexes 117

Table 6.1 Cyclic Voltammetry Results for Phenanthrolines 128

Table 6.2 Spectroscopic Data and Proposed Assignments

for Phenanthrolines 136

Vlll

Table 7.1 Electrochemical Data for Catenanes and Their Precursors . . . . . . . . . . 146

Table 7.2 Spectroscopic Data and Proposed Assignments

for Catenanes and Their Precursors 154

IX

LIST OF ILLUSTRATIONS

PAGE

Fig. 1.1 Energy level diagram for an octahedral complexes containing ligands with K-

orbitals. From Bryant, G. M.; Fergusson, J. E.; Powell, H. K. J. Aust. J. Chem. 1971,

24,257 8

Fig.2.1 The OTTLE cell used in spectroelectrochemical studies (After Song, J-I. PhD.

Thesis, University of Glasgow, 1989) 14

Fig.2.2 Some aza-biphenyl compounds which are discussed in Chapter III 17

Fig.2.3a Platinum(II) complexes which are discussed in Chapter III 18

Fig.2.3b Platinum(II) complexes which are discussed in Chapter III 19

Fig.2.4a [Ru(n)(bipy)2(l,l'-biiq)]2+ and related species which aure discussed in Chapter

IV 20

Fig.2.4b Ruthenium(II) complexes which are discussed in Chapter IV 21

Fig.2.4c Ruthenium(II) complexes which are discussed in Chapter IV 22

Fig.2.5 Platinum(II) and palladium(II) complexes which are discussed in Chapter V. .23

Fig.2.6 Phenanthrolines discussed in Chapter VI 24

Fig.2.7a Catenanes and their precursors which are discussed in Chapter VII 25

Fig.2.7b Catenanes and their precursors which are discussed in Chapter VII 26

Fig.3.1 Cyclic voltammogram of 2,4'-bipyridine in DMF (supporting electrolyte 0.1 M

TBAPF6, scan rate 500 mV/s, at room temperature) 29


TBAPF6, scan rate 4.0 V/s, at room temperature) 30

Fig.3.3 Cyclic voltammogram of N'-methyl-2,4'-bipyridine in DMF (supporting

electrolyte 0.1 M TBAPF6, scan rate 200 mV/s, at room temperature) 33

Fig.3.4 Cyclic voltammogram of 4,4'-diphenyl-2,2'-bipyridine in DMF (supporting

electrolyte 0.1 M TBAPF6, scan rate 500 mV/s, at room temperature) 34

Fig.3.5 The plot of the cathodic current of 4,4'-diphenyl-2,2'-btpyridine against the

square root of scan rate 35

Fig.3.6 Cyclic voltammogram of [P^bipy^lJ in DMF (supporting electrolyte 0.1 M

TBAPF6, scan rate 0.2 V/s at room temperature) 38

Fig.3.7 Cyclic voltammogram of [Pt^bipy^lJ in DMF (supporting electrolyte 0.1 M

TBAPF6, scan rate 2 V/s at room temperature) 39

Fig.3.8 Cyclic voltammogram of [Pt(bipy)(4-NCpy)Cl)]+ in DMF with supporting

electrolyte, 0.1 M TBAPF6, scan rate 2 V/s, at room temperature 40

Fig.3.9 Cyclic voltammogram of [Pt(ph2-bipy)Cy in DMF with supporting electrolyte,

0.1 M TBAPF6; scan rate 0.2 V/s; at room temperature 42

Fig.3.10 Cyclic voltammogram of [Pt(Me2-bipy)ClJ in DMF with supporting

electrolyte, 0.1 M TBAPF6; scan rate 0.5 V/s; at room temperature 43

Fig.3.11 Cyclic voltammogram of Pt(2,4'-bipyOct-H)Cl2 in DMF with supporting

electrolyte, 0.1 M TBAPF6; scan rate 200 mV/s; at room temperature 44

Fig.3.12 Cyclic voltammogram of Pt(2,4'-bipyOct)Cl3 in DMF with supporting

electrolyte, 0.1 M TBAPF6; scan rate at 200 mV/s; at room temperature 45

XI

Fig.3.13 Cyclic voltammogram of [Pt(2,4'-bipyMe-H)(bipy)]2+ in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature, scan rate 2 V/s 48

Fig.3.14 Cyclic voltammogram of [Pt(2,4'-bipyMe-H)(py)J2+ in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature, scan rate 0.2 V/s 49

Fig.3.15 Cyclic voltammogram of [Pt(terpy)Cl]+ in DMF with supporting electrolyte

0.1 M TBAPF6 at room temperature, scan rate 0.5 V/s 50

Fig.3.16 Cyclic voltammogram of 2,2':6",2"-Terpyridine in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature 51

Fig.3.17 Cyclic voltammogram of [PtDCMBClJ in DMF with supporting electrolyte

0.1 M TBAPF6 scan rate 500 mV/s; at room temperature 52

Fig.3.18 7t-Orbital diagram for biphenyl 56

Fig.3.19 Electronic absorption spectra of 2,4'-bipyridine and its one electron reduction

product in DMF (c = 6.6 x 10"4 M) with 0.1 M TBAPF6; parent (dashed line) and

singly reduced form (solid lines) 57

Fig.3.20 Electronic absorption spectra of N'-methyl-2,4'-bipyridinium and its one

electron reduction product in DMF (c = 6.4 x 10"4 M) with 0.1 M TBAPF6; (Solid line

— parent; dashed line — singly reduced) 58

Fig.3.21 Electronic absorption spectra of [Pt(2,4'-bipyOctyl)Cl3] and its one electron

reduction product in DMF (c = 7.8 x 10"4 M) with 0.1 M TBAPF6 (Solid line —

parent; Dashed line — singly reduced) 59

Fig.3.22 Electronic absorption spectra of [Pt(2,4'-bipyOctyl-H)ClJ and its one electron

reduction product in DMF (c = 5.7 x 10"4 M) with 0.1 M TBAPF6 (Solid line —

xu

parent; Dashed line — singly reduced) 64

Fig.3.23 Electronic absorption spectra of [PtCbipy^lJ and its one electron reduction

product in DMF (c = 2.3 x 10 3 M) with 0.1 M TBAPF6 67

Fig.3.24 Electronic absorption spectra of [Pt(bipy)(4-CNpy)Cl]+ and its one electron

reduction product in DMF (c = 1.2 x 10"3 M) with 0.1 M TBAPF6 70

Fig.3.25 Electronic absorption spectra of [PtDCMBClJ and its one electron reduction

product in DMF (c = 2.3 x 10"4 M) with 0.1 M TBAPF6 71

Fig.4.1 Cyclic voltammogram of l,l'-biisoquinoline in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature; scan rate 0.5 V/s 78


electrolyte 0.1 M TBAPF6 at room temperature, scan rate 4 V/s 79







Fig.4.6 Cyclic voltammogram of 2,2'-bisquinoline in DMF with supporting electrolyte

0.1 M TBAPF6 and ferrocene as internal standard at room temperature; scan rate 0.5

V/s; from 1.0 to -3.0 V 83


0.1 M TBAPF6 at room temperature; scan rate 0.5 V/s; from 1.0 to -2.5 V 84

Xlll


0.1 M TBAPF6 at room temperature; scan rate 4 V/s 85

Fig.4.9 Cyclic voltammogram of [Ru(biiq)(bipy)2]2+ in DMF with supporting


Fig.4.10 Cyclic voltammogram of [Ru(biiq)(bipy)J2+ in CH3CN with supporting


Fig.4.11 Cyclic voltammogram of YoungOl in DMF with supporting electrolyte 0.1 M

TBAPF6 at room temperature; scan rate 0.5 V/s 90

Fig.4.12 Cyclic voltammogram of Young02 in DMF with supporting electrolyte 0.1 M

TBAPF6 at room temperature; scan rate 0.5 V/s; with FeCp2 as internal standard . 91





Fig.4.15 Electronic absorption spectra of l,l'-biisoquinoline and its one electron

reduction product in DMF (c = 2.2 x 10"4 M) with 0.1 M TBAPF6; parent (solid line)

and singly reduced form (dashed lines) 96

Fig.4.16 Electronic absorption spectra of [Ru(biiq)(bipy)J2+ and its one electron



Fig.4.17 Electronic absorption spectra of youngOl and its one electron reduction

product in DMF (c = 1.3 x 10"3 M) with 0.1 M TBAPF6; parent (solid line) and singly

xiv

reduced form (dashed lines) 101

Fig.4.18 Electronic absorption spectra of young02 and its one electron reduction







product in DMF (c = 1.2 x 10'3 M) with 0.1 M TBAPF6; parent (solid line) and singly


Fig.5.1 Cyclic voltammogram of [(bipym)Pt(CH2SiMe3)J in DMF with supporting

electrolyte 0.1 M TBAPF6; scan rate 0.5 V/s; at room temperature 110

Fig.5.2 Cyclic voltammogram of [(bipym)Pt(CH2SiMe3)2] in DMF with supporting

electrolyte 0.1 M TBAPF6; scan rate 2 V/s; at room temperature I l l

Fig.5.3 Cyclic voltammogram of [(bipym)Pd(CH2SiMe3)J in DMF with supporting

electrolyte 0.1 M TBAPF6; scan rate 1 V/s; at room temperature 112

Fig.5.4 Cyclic voltammogram of [(Me3SiCH2)2Pd(bipym)Pt(CH2SiMe3)2] in DMF with

supporting electrolyte 0.1 M TBAPF6; scan rate 0.5 V/s; at room temperature . . . 115

Fig.5.5 Cyclic voltammogram of [(Me3SiCH2)2Pd(bipym)Pd(CH2SiMe3)2] in DMF with

supporting electrolyte 0.1 M TBAPF6; scan rate 0.2 V/s; at room temperature . . . 119

Fig.5.6 Electronic absorption spectra of [(bipym)Pt(CH2SiMe3)J and its one electron


xv


Fig.5.7 Electronic absorption spectra of [(bipym)Pd(CH2SiMe3)J and its one electron



Fig.5.8 Electronic absorption spectra of [(Me3SiCH2)2Pd(bipym)Pt(CH2SiMe3)2] and its

one and two electron reduction product in DMF (c = 7.5 x 10"4 M) with 0.1 M

TBAPF6; parent (solid line) and singly reduced form (dashed lines 122

Fig. 6.1 Cyclic voltammogram of 1,7-phenanthroline in DMF with supporting






Fig. 6.4 Cyclic voltammogram of 1 -methyl-1,10-phenanthrolinium in DMF with

supporting electrolyte 0.1 M TBAPF6 at room temperature; scan rate 0.2 V/s . . . 132

Fig. 6.5 Cyclic voltammogram of l-methyl-l,10-phenanthrolinium in DMF with


Fig. 6.6a Cyclic voltammogram of 7-methyl-4,7-phenanthrolinium in DMF with


Fig. 6.6b Cyclic voltammogram of 7-methyl-l,7-phenanthrolinium in DMF with


Fig. 6.7 Electronic absorption spectra of 1,7-phenanthroline and its one electron

xvi



Fig. 6.8 Electronic absorption spectra of 4,7-phenanthroline and its one electron



Fig. 7.1 Cyclic voltammogram of [BBEPYBIXYCY]4* (JFS08) in Acetonitrile with

supporting electrolyte 0.1 M TBAPF6 at room temperature; scan rate 1 V/s . . . . 147

Fig. 7.2 Cyclic voltammogram of [BBIPYXY]4+ (JFS06) in Acetonitrile with


Fig. 7.3 Cyclic voltammogram of [BBIPYBIXYCY]4* (JFS07) in Acetonitrile with


Fig. 7.4 Cyclic voltammogram of JFS03 in Acetonitrile with supporting electrolyte


Fig. 7.5 Cyclic voltammogram of JFS04 in Acetonitrile with supporting electrolyte


Fig. 7.6 Cyclic voltammogram of JFS05 in acetonitrile with supporting electrolyte 0.1

M TBAPF6 at room temperature; scan rate 2 V/s 152

Fig. 7.7 Electronic absorption spectra of [BBIPYBIXYCY]4* (JFS08) and its two-

electron reduction product in Acetonitrile with 0.1 M TBAPF6 (c = 6.3 x 10"4 M) 157

Fig.7.8 Electronic absorption spectra of two-electron reduced [BBIPYBIXYCY]4*

(JFS08) in Acetonitrile with 0.1 M TBAPF6 (c = 6.8 x 10"4 M) 158

Fig.7.9 Electronic absorption spectra of [2]catenane (JFS05) and its two-electron

xvii

reduction product in acetonitrile with 0.1 M TBAPF6 (c = 3.5 x 10"4 M) 159

Fig.7.10 Electronic absorption spectra of [2]catenane (JFS04) and its two-electron

reduction product in Acetonitrile with 0.1 M TBAPF6 (c = 5.0 x 10"4 M) 160

Fig.7.11 Correlation between the band at 262 nm and 401 nm of JFS05 in the process

of reduction 161

Fig.7.12 Correlation between the band at 262 nm and 615 nm of JFS05 in the process

of reduction 162

Fig. 8.1 Individual atomic orbital contributions to the LUMO of co-planar biphenyl.

(From Mopac Version 6.00 calculation) 167

Fig.8.2 Individual atomic orbital contributions to the LUMO of phenanthrene. (From

Mopac Version 6.00 calculation) 168

xvm

CHAPTER I

INTRODUCTION

This dissertation is devoted to the characterization of a remarkable and

interesting family of electrochemically and spectroscopically active supramolecules

with an azabiphenyl as active moiety. Their electronic structure has been closely

defined by comparative examination of their redox potentials, and their electronic

absorption spectra in their original forms and also, where possible, in their reduced or

oxidized forms. After initial electrochemical characterization by cyclic voltammetry,

the reduced and oxidized species were generated electrochemically inside an optical

transparent thin layer electrochemical (OTTLE) cell by controlled potential electrolysis

and their electronic spectra were taken in situ.

1.1 Frontier Molecular Orbitals

In modem chemistry, the electronic structure of a chemical system, an atom, an

ion or a molecule, is to a good approximation expressed in terms of an array of two-

electron "orbitals". These electronic orbitals are ordered in energy, and are filled

progressively by electrons from lower energy to higher, to form the ground state

electronic configuration. Of these electronic orbitals, the most important orbitals in

determining the chemistry of a particular chemical system are the HOMO and LUMO.

The HOMO (Highest Occupied Molecular Orbital) is occupied last when electrons fill

the available orbitals from lower energy to higher energy. The LUMO (Lowest

1

2

Unoccupied Molecular Orbital) is the next higher orbital after the HOMO. HOMO

and LUMO are collectively called frontier orbitals, because in general, chemical

processes start with the interaction and reorganization of the frontier orbitals, HOMOs

and LUMOs, of the participating chemical systems. The chemical properties of a

chemical system are largely defined by its frontier orbitals.

By reduction or oxidation, electrons can be added into or removed from a

molecule. The electron added will go into the LUMO first, and the electron removed

first will come from the HOMO. The reduction and oxidation potentials which tell

how easily the chemical system under study can be reduced or oxidized, can be easily

measured electrochemically. The redox potentials provide measurements of the energy

levels of the HOMO and LUMO. Electronic spectroscopy is another probe to look

into the electronic structure of a chemical system. In the electronic transition process,

an electron in a HOMO or near HOMO orbital gets energy from a photon, and

promotes itself into an orbital with higher energy, leaving a hole in the original orbital,

thus forming the excited state. If we neglect changes in electron-electron repulsion

energy, the position of the bands in the electronic spectra is a measure of the energy

difference between the two participating orbitals. This is of course a drastic over-

simplification, but the values obtained may still be useful for comparisons within a

group of similar molecular species. Because the mass of an electron is much smaller

than that of a nucleus, and the time scale for an electronic transition is much shorter

than that for the vibrational relaxation needed by geometry adjustment, the shape of

the molecule is frozen at its ground state in the electronic transition process, (Franck-

3

Condon Principle), so that usually the electronic spectra are broad and sometimes

show vibrational coupling structure. In electrochemical reduction and oxidation,

however, the species has time to adopt its equilibrium geometry in both oxidation

states.

The molecular orbitals are intrinsically delocalized; all orbitals of the atoms

making up the molecule are more or less fused together by overlapping to form the

molecular orbital. But in many cases, the localized molecular orbital model is still

suitable and more descriptive in practice. In the localized molecular orbital model, the

orbitals can be described as based on each individual fragment For example, in

coordination compounds, they can be classified as metal-based d orbitals and ligand-

based a and K orbitals. In this way, the overlapping of the orbitals from different

parts of the molecule is ignored (zeroth-order approximation). Under this localized

molecular orbital model, the electronic absorption bands of complexes can be

classified accordingly into metal-based, ligand-based, and charge transfer bands.

Transitions between molecular orbitals mainly localized on the metal center are termed

metal-based transitions, they are generally d -» d transitions for transition metals even

though some time s -» p and p -¥ d transitions are observed. Transitions between

molecular orbitals mainly localized on the ligands are termed ligand-based transitions;

they are mainly % -» 7t* transitions in aromatic ligands such as those under study. In

other cases a -» a*, n a* and n -> ji* transitions also exist. Transitions between

molecular orbitals localized at different parts of a molecule which cause the

redistribution of electronic charge are called charge transfer transitions. More

4

specifically, in coordination complexes, the charge transfer transitions can be divided

into metal to ligand charge transfer, MLCT, ligand to metal charge transfer, LMCT,

and in some cases ligand to ligand charge transfer. MLCT is usually observed when

there is an oxidizable metal center. Sometimes it is also called as metal oxidation

charge transfer because it has the metal center oxidized to a higher oxidation state in

the excited state compared with the ground state. In the same way, LMCT can also be

called metal reduction charge transfer, and is often observed when the metal center is

reducible. Under the localized electronic Orbital model, the reduction of a

coordination compound can be classified as a ligand-based reduction if the added

electron resides in a ligand-based orbital, or a metal-based reduction if the added

electron resides in a metal-based orbital; similarly for oxidation.

The positions of electronic transition bands reflect the energy difference

between the molecular orbitals involved. The intensity of a band is largely governed

by the selection rules. For the electronic absorption, which is mainly from an

electronic dipole transition, the selection rule requires that the system has a transition

moment between the ground and excited states under the influence of an electric

dipole so that it can interact with the oscillating electric vector and get energy

transferred. For an electronic absorption band to have significant intensity, it must

arise from an allowed transition. Usually some n -» k and many charge transfer

transitions are orbitally allowed. For an octahedral complex, the metal-based d -» d

transitions are orbitally forbidden, so they usually are very weak.

The observation of the changes in redox potential and in the energy of

5

electronic absorption bands for a given component in different molecular environments

can be used as a way to investigate the interaction between this component and its

environment. The electron transfer absorptions are absent from the individual

components, and directly result from transitions between molecular orbitals localized

at different components of the supramolecular system.

1.2 Supramolecules

The localized molecular orbital theory is the fundamental theory for

supramolecular chemistry. Supramolecular chemistry studies the chemistry of systems

(supramolecules) made up of molecular components in the same way as molecules are

made up of atoms.1 A few examples of supramolecules are catenanes, rotaxanes, and

donor-acceptor complexes. Most coordination complexes can also fit into this

category, regarding metal and ligands as the building blocks.

The term supramolecule is used here to refer to a concept rather than a certain

category of molecules. The name of supramolecule is given to certain substances, not

because they are large in size, but because their properties under examination can be

understood based on the localized electronic theory. The features of a supramolecule

can be pictured according to its individual molecular components, whose behavior as a

part of a supramolecule is the direct development from those of the isolated parts or of

suitable model compounds. It is no surprise that those properties are more or less

modified when the molecules are organized into a supramolecular system, but in many

cases they can be understood based on the intrinsic properties of the components,

considering those modifications which result from interactions within supramolecular

6

systems as perturbations. The interactions among atoms within classical molecules are

mainly ionic and covalent interactions. In addition to these, in supramolecular

chemistry, there are also donor-acceptor, dispersion dipolar, hydrophobic, and

sometimes hydrogen bonded interactions which are not of primary importance in the

study of the intermolecular interactions for classical molecules but are important for

supramolecules.

1.3 Azabiphenyl Systems

The chemical systems under study are those containing polypyridine or

phenanthroline moieties. 2,2'-Bipyridine and 1,10-phenanthroline, as two members of

this family, have been well known for more than a hundred years as effective

bidentate ligands. The red-colored ferrous salt of 2,2'-bipyridine was first reported by

Blau in 1888.2 In 1898, Blau reported the synthesis of 1,10-phenanthroline and

demonstrated its similarity to 2,2'-bipyridine. He also discovered the reversible nature

of oxidation of the iron(II) complexes.3 In 1912 Werner demonstrated an octahedral

configuration for the tris-2,2'-bipyridine iron(II) cation by successful resolution of its

optical forms.4 In 1928, Manchot and Lehmann used 2,2'-bipyridine to study the

reaction of ferrous iron with hydrogen peroxide.5 Feigl and Hamburg described a

similar qualitative application three years later® Apparently, Bode was the first person

to use the reagent for quantitative purposes, determining iron in beer.7 Widespread

interest in the analytical applications of these bipyridine and phenanthroline complexes

developed in 1931 when Hammett, Walden and Chapman described the use of the

iron(II) complexes as reversible, high potential oxidation-reduction indicators.8

7

Studies of the crystal structure of bipyridine reveal that the two pyridine rings

are nearly coplanar with N-atoms in the trans configuration.9 In solution, dipole

moment measurements indicate that the molecule is approximately planar and also in

the trans arrangement.10"12 The cisoid form, undoubtedly is adopted for chelate ring

formation with metal ions; and with properties comparable to phenanthroline, the five-

membered chelate ring is most probably very close to coplanar with the rest of

bipyridine molecule.

The electronegativity of nitrogen is higher than that of carbon, so the n*

orbitals of bipyridines and phenanthrolines are lower in energy than that of biphenyl

and phenanthrene, their all-carbon analog, and they can function as good n acceptor

ligands so as to stabilize low oxidation states of the metal center. These bipyridine

and phenanthroline complexes are rich in spectroscopy, with bands in the ultraviolet,

visible and sometimes the infrared region, because of the lower it* orbital of bipyridine

and phenanthroline. The electronic absorption bands of the complexes directly relate

to their electronic structure, their frontier orbitals. All of these complexes are also

electrochemically reducible under suitable experimental conditions.

1.4 Symmetry Considerations

Six coordinated tris-complexes of bidentate ligands such as M(bipy)3 or

M(phen)3 have D3 symmetry. When one or two of these bidentate ligand is substituted

by other monodentate ligands, the symmetry of the molecule is changed to G, for cis-

[M(bipy)2XJ, Q for cis-[M(bipy)2XY], D2h for trans-[M(bipy)2XJ, and for trans-

[M(bipy)2XY]. Mono-bipyridine complexes belong to C,v for [M(bipy)X4], or C[

•

' l

\

(n + l)p / ~2 l̂ \

—f '1 rm -x

9

for [M(bipy)X2Y2J. In an Oh environment, if the metal ion coordinates with six

identical monodentate ligands, the metal-based d orbitals will split into two groups,

dxy, dyz and dzx with lower energy belonging to t,g and dz2 and dl2.y2 with higher energy

belonging to eg. There are 12 ligand-Jt orbitals for the octahedral complexes. They

split into tlg, tlu, tjg and 4 sets of group orbitals in an octahedral field. Among the

4 sets of group orbitals, tlg, and t2u are nonbonding in character, and tlu interacts with

(n+l)p. Only will interact with metal center's d(tjg) orbitals to form % bonds. The

interaction between metal t2g and ligand n* orbitals is an important factor governing

the chemistry of metal complexes. The simplified energy-level diagram is pictured as

Fig. 1.1. Tris-bidentate metal complexes have D3 symmetry. There are only three 7t

orbitals corresponding to the ligand HOMOs, and three for the LUMOs. Their

chemistry can often be discussed under Oh group symmetry. While doing this, one

should be aware that the D3 group is only a sub-group of Oh and it is not strictly

correct to consider the metal d-orbitals as and eg as the degeneracy will be reduced

by the lower symmetries. Under D3 symmetry the t,g orbitals will split into a,+e,

while the tlg orbitals split into a ^ e . The energy gaps between those a and e orbitals

are not large but some believe that they are sources of structure on the intraligand and

charge-transfer bands of the tris-bipyridine complexes.14,15 Comparing with tris-

complexes, the lowering of symmetry on going to bis- and mono-bipyridine

complexes, is even less significant, and their spectra (in solution and room

temperature) do not differ in band structure to any great extent from those of the tris-

complexes. The same arguments are also valid for the four-coordinated complexes of

10

bipyridines. If they adopt planar geometry, they can be treated under D4h symmetry.

REFERENCES

(1) Stoddart, J. F. Chem. Brit. 1988, 24, 1203.

(2) Blau, F. Ber. 1888, 21, 1077.

(3) Blau, F. Monatsh, 1898, 19, 647.

(4) Werner, A. Ber. 1912, 45, 433.

(5) Manchot, W.; Lehmann, G. Ann. Chem. 1928, 460, 191.

(6) Feigl, F., Hamburg, H. Z. Anal. Chem. 1931, 86, 7.

(7) Bode, B. Wochschr. Brau. 1933, 50, 321.

(8) Walden, G. H. Jr.; Hammett, L.P.; Chapman, R. P. J. Am. Chem. Soc., 1931,

53, 3908.

(9) Cagle, F. W. Jr. Acta Cryst. 1948, 1, 158.

(10) Fielding, P. E.; Lefevre, R. J. W. J. Chem. Soc., 1951, 1811.

(11) Cumper, C. W. N.; Giaman, R.F.A.; Vogel, A. I. J. Chem. Soc., 1962, 1188.

(12) Cureton, P. H.; Lefevre, C. G.; Lefevre, R. J. W. J. Chem. Soc., 1963, 1736.

(13) Bryant, G. M.; Fergusson, J. E.; Powell, H. K. J. Aust. J. Chem. 1971, 24, 257.

(14) MacCaffery, A. J.; Mason, S. F.; Norman, B. J. J. Chem. Soc. (A), 1969, 1428.

(15) Ferguson, J.; Hawkins, C. J.; Kane-Maguire, N. A. P.; Lip, H. Inorg. Chem.,

1969, 8, 771.

11

CHAPTER H

EXPERIMENTAL

2.1 Materials

The solvents used in electrochemistry and spectroelectrochemistry were HPLC

grade acetonitrile or DMF, purchased from Aldrich. The acetonitrile was distilled

twice over phosphorus pentoxide, and DMF twice over calcium hydride before use.

All distillations were carried out under an atmosphere of nitrogen which was dried

through a molecular sieve column. The freshly distilled solvent was transferred to the

electrochemistry cell with a syringe. The supporting electrolyte was either tetra-n-

butylammonium tetrafluroborate or hexafluorophosphate, which were purchased from

Aldrich and pre-dried in an oven at 120°C for about 2 to 4 hr. before being used.

Before each set of experiments, the cyclic voltammogram of blank solvent with

supporting electrolyte was collected to confirm that their quality was satisfactory.

2.2 Electrochemistry and Spectroelectrochemistry

The electronic spectra were collected with a Lambda 9 UV-VIS-NIR

spectrometer made by Perkin Elemer Corporation. The electrochemical experiments

were performed with an EG & G Princeton Applied Research Potentiostat Model 273

Potentiostat/Galvanostat, controlled by an IBM compatible computer with

Electrochemical Analysis Software V 4.01 Beta. Cyclic voltammograms were

recorded at room temperature, the results being saved as electronic files and displayed

12

13

or plotted out when needed.

Cyclic voltammetry experiments were performed under an atmosphere of argon

which was dried by passing through a column filled with 4 A molecular sieve and

then saturated with the appropriate solvent. The same argon gas was also used to

purge all solutions prior to experimentation. The working concentration of analyte

was usually around 10"3 M with 0.1 M tetrabutylammonium tetrafluroborate or

hexafluorophosphate as supporting electrolyte. The routinely used scan rate was 0.5

V/s, but the results were verified by changing the scan rate over the range 20 mV/s to

4.0 V/s, which in no case caused significant shift of the peak potentials. The

electrochemical system used here was a standard three-electrode system. The cell used

for cyclic voltammetry was a single-compartment-three-electrode cell bought from EG

& G Company. The working electrode was the cross-section of a platinum wire 0.368

mm in diameter which was sealed in a glass tube, and the counter-electrode was a

piece of the same platinum wire about 5 mm in length. The reference electrode was

Ag/0.01M-AgN03 in a suitable solvent with 0.09 M supporting electrolyte. It was

connected to the bulk solution through a salt bridge of 0.1 M supporting electrolyte in

the working solvent with porous Vycor frits. Potentials were reported against the half-

wave potential for the oxidation of ferrocene, which was added at the end of each run

as an internal standard; the peak to peak separation for the standard in all cases was

within the range 60-80 mV. All the glassware and supporting electrolyte used were

pre-dried in an oven at about 120°C and cooled down to room temperature inside a

desiccator before being used. The working electrode was polished with sand paper,

14

and all the platinum electrodes were washed with concentrated nitric acid and then

rinsed with water and properly dried before being used.

Spectroelectrochemical experiments were carried out at room temperature.

When appropriate, the same solution used in cyclic voltammetry was used for

spectroelectrochemical experiments, sometimes diluted.

Spectroelectrochemical experiments were carried out in a specially designed

quartz cell with one millimeter optical path length, as shown in Fig. 2.1. It is also a

three-electrode electrochemical system with the same reference electrode as used in the

cyclic voltammetry experiment. The working electrode is a platinum gauze served as

an Optically Transparent Thin-Layer Electrode, or OTTLE in short, which is mounted

in the cell across the light beam of the spectrometer. The counter-electrode here is a

platinum gauze isolated from the bulk solution with a porous Vycor frit to avoid

contaminating the main cell by its electrochemical products. The spectrum of the

parent species was collected first, and the potential of the working electrode was then

set at the desired position and spectra of the reduced species were collected

continuously, until there were no further changes when the reduction was regarded as

complete. Extinction coefficients were caCulated for the electrochemically generated

species assuming quantitative conversion. Generally, the regeneration of parent

material was followed spectroscopically, and the isosbestic points during reduction

were also inspected, to check for decomposition or side-reactions. The reference cell

used while collecting the electronic absorption spectra was a standard 1 mm quartz

cell, filled with the same solvent with supporting electrolyte used for working solution

15

Platinum gauze counter electro

1 mm UV cell

Ag/Ag+ reference electrode

orous vycor frits

Platinum gauze working electrode

Fig.2.1 The OTTLE cell used in spectroelectrochemical studies (After Song, J-I. PhD.

Thesis, University of Glasgow, 1989)

16

and containing a similar piece of platinum gauze for background correction.

2.3 The Compounds Under Examination

The structures of the chemical systems under examination are illustrated in the

following diagrams.

In Fig. 2.2, 2,2'-bipyridine, 2,4'-bipyridine and 4,4'-diphenyl-2,2'-bipyridine

were purchased from Aldrich. All platinum(II) complexes shown in Fig. 2.3, were

kindly supplied by F. Wimmer of Universiti Brunei Darussalam, Bansar Seri Begawan,

Brunei. In Fig. 2.4, 2,2'-bisquinoline was purchased from Aldrich, and 1,1'-

bisisoquinoline and [Ru(II)(bipy)2(l,r-biiq)]2+ (l,l'-biiq = l,l'-biisoquinoline) were

kindly supplied by M. T. Ashby of The University of Oklahoma. The Ru(II)

complexes in Fig. 2.4 and the platinum(II) and palladium(II) complexes in Fig. 2.5

were supplied by G. B. Young of Imperial College of Science, Technology and

Medicine, London, UK. The catenanes and their precursors in Fig 2.7 were supplied

by J. F. Stoddart of The University, Sheffield, UK.

2.4 Molecular Orbital Calculations

Molecular orbital calculations were performed on a UNIX platform with a

MOP AC package, version 6.00, by Frank J. Seiler Research Laboratory, U.S. Air

Force Academy. The Hamiltonian used was PM3.

17

2,2'-bipyridine 2,4'-bipyridine

N'-methyl-2,4'-bipyridinium N'-methyl-2,2'-bipyridinium

N'-octyl-2,4'-bipyridinium 4,4'-diphenyl-2,2'-bipyridine

Fig. 2.2 Some aza-biphenyl compounds which are discussed in Chapter DI

18

N. el \ / Pt(II)

CI

rr Pt(II) /\ N CI

CN

[Pt(bipy)Cl2]

Pt(II)

[Pt(bipy)(4-NCpy)Cl]-t

Pt(II)

[Pt(ph2-bipy)Cl2] [Pt(Me2-bipy)Cl2]

Pt(II) Pt(II)

[Pt(2,4' -bipyOc t-H)Cl2]

N I Oct

[Pt(2,4'-bipyOct)Cl3]

Fig. 2.3a Platinum(II) complexes which are discussed in Chapter IE

19

Pt(II) Pt(II)

[Pt(2,4'-bipyMe-H)(bipy)]2+ [Pt(2,4'-bipyMe-H)py2P+

— p t d D i I CI

[Pt(terp)Cl]+

Pt(II)

[Pt(DCMB)Cl2]

Fig.2.3b Platinum(II) complexes which are discussed in Chapter in

20

1,1' -biisoquinoline

2,2'-bisquinoline

N ii— Ru (11) •

[Ru(II)(bipy )2( 1,1' -biiq)]2+

Fig.2.4a [Ru(n)(bipy)2(l,l'-biiq)]2+ and related species which are discussed in Chapter IV

21

t-Bu P(OCH3)3

t-Bu

Ru(II)

CH2-^.Si

P(OCH3 ) 3

[(bipy')Ru[P(OCH3)3]2CH2CH2SiMe3 ] (YoungOl)

t-Bu

t-Bu

P(OCH3)3

t 4*CH2SIMe3 Ru (II)

P(OCH3)3

CH2SIMe3

[(bipy')Ru[P(OCH3)3]2(CH2SiMe3)23(Young02)

Fig.2.4b Ruthenium(II) complexes which are discussed in Chapter IV

22

t-Bu.

t-Bu

P(OCH2CH3) 3

t %%CH2SIMe3 Ru (II]

P{OCH2CH3)3

CH2SIMe3

[(bipy')Ru[P(OCH2CH3)3]2(CH2SiMe3)23(Young03)

P(OCH2CH3)

t*CH2SIMe3 Ru(II)

^P(OCH2CH3)3

CH2SIMe3

[(Me2-bipy)Ru[P(OCH2CH3)3]2(CH2SiMe3)2](Young04)

Fig.2.4c Ruthenium(II) complexes which are discussed in Chapter IV

23

n \ yCH;-SiMe:. yCH2-SiMe3

I / V 1 1 ' I M < n >

^ \ / CH2-SiMe, NCH2-SiMe,

[(bipym)Pt(CH2SiMe3)2] [(bipym)Pd(CH2SiMe3)2l

r i , ^ N v

J Me3Si-CH2v / yCH2-SiMe3

Pd(ll) I .Pt(II) Me3Si-CH2'

r \ / CH2-SiMe3 NT

u

[(Me3SiCH2)2Pd(bipym)Pt(CH2SiMe3)2]

i f ^ i

Me3Si'CH2v / S

yCH2"SiMe3 Pd(II) I p d ( H )

Me3Si-CH2^ CH2-SiMe3

U

[(Me3SiCH2)2Pd(bipym)Pd(CH2SiMe3)2]

Fig.2.5 Platinum(II) and Palladium(II) complexes which are discussed in Chapter V

24

1,10-phenanthroline 4,7 -phenanthroline

1,7 -phenanthroline CH3

N-methyl-1,10-phenanthrolinium

7-methyl-1,7-phenanthroline

Fig.2.6 Phenanthrolines discussed in Chapter VI

25

JFS-01 JFS-03

o ^ r c P o ^ „

JSF-02

/ v t s / s V o o o o o

r - \ / o o

6

+ 0

w U

0

A _ / °

JFS-04

Fig.2.7a Catenanes and their precursors which are discussed in Chapter VII

26

JFS-05

n n n o o o a o o o

o o o q o o c V - / W W U \ ( w

JFS-06 JFS-07

rO~On 8

JFS-08 ( - H Z Z H - j = c u p ^ ^ ^

(—c=>—| = ^ ~ V ^ | CH,

? - r\J~ f

Fig.2.7b Catenanes and their precursors which are discussed in Chapter VII

CHAPTER m

ELECTRO- AND SPECTROELECTROCHEMICAL STUDIES OF PLATINUM(II)

BIPYRIDINE COMPLEXES AND RELATED SPECIES

Bidentate aza-biphenyl and terphenyl ligands readily form stable complexes

with platinum(II). Such complexes have been extensively studied in coordination and

organometallic chemistry.1"20 Some of them also show significant functions in

biochemistry-related processes. Some cis-platinumammine compounds act as

antitumor drugs by binding covalently to DNA under specific conditions.21 The

preparation of platinum(II) 2,2'-bipyridine complexes was first reported by Morgan

and Burstall about sixty years ago.1 2,2'-Bipyridine is an important bidentate ligand

which forms complexes with most of the transition metals. It also can be converted to

a monodentate ligand by quaternizing one-of the nitrogens, resulting in a cation which

is isoelectronic with 2-phenylpyridine.22"26 The resulting cation can undergo ortho-

metallation at the C(3) position of the ring being quatemized to become the bidentate

zwitterionic ligand (Mebipy - H) which is isoelectronic with bipyridine.27'28 All of

these platinum(II) complexes have very rich electrochemistry and informative spectra.

Their electrochemical and spectral properties have been investigated in detail.

2,4'-Bipyridine differs from 2,2'-bipyridine as its two nitrogens are located at

unsymmetrical positions, and it can not serve as a chelating bidentate ligand with two

nitrogen atoms as donors bonding with one metal center. But when it coordinates

27

28

through the nitrogen on the 2-pyridine ring, it can undergo orthometallation at the C(3)

position of the 4-pyridine ring, forming complexes with an anionic ligand which is

isoelectronic with the bipy-H anion. By quaternizing on the 4-pyridine ring, we can

make a cationic species which can coordinate with metal ions as the same way as

above, but as a zwitterionic bidentate ligand isoelectronic with the bipyridines.

3.1 Electrochemistry of Bipyridines and Platinum(II) Complexes

3.1.1 Electrochemistry of Free Bipyridines

The electrochemistries of 2,2'-bipyridine and 4,4'-bipyridine have been

investigated before,29 but there are no reports concerning 2,4'-bipyridine.

3.1.1.1 Electrochemistry of 2,4'-Bipyridine

The cyclic voltammogram of 2,4'-bipyridine as shown in Fig.3.1 and Fig.3.2

has two accessible reductions in DMF. The first one at -2.50 V (vs. FeCp2 /FeCp2+) is

a chemically reversible one electron reduction. As with 2,2'-bipyridine,29 the second

reduction of 2,4'-bipyridine is chemically irreversible at the scan rate up to 4 V/s.

Brown and Butterfield,32 explained this by suggesting that the electrochemically

formed dianion extracts a proton from the tetrabutylammonium cation leading to its

own degeneration and the formation of butene and tributylamine. In the biphenyl

system, the 7t(7) orbital has larger electron densities at para than at ortho positions, so

the nitrogen, whose electronegativity is higher than carbon, will stabilize the 7t(7)

orbital better when it is at a para position than when at an ortho position. That is why

the potential of the first reduction of 2,4'-bipyridine lies between the first reduction

potential of 2,2'-bipyridine (-2.56 V) and 4,4'- bipyridine29 (-2.40 V) under the same

29

1 I I 1 o o o o o o o o o o © o

cu © OJ 1

S

tu

(Vn) i


TBAPF6, scan rate 500 mV/s, at room temperature)

30

» SI 5 2

o 8

o o

OJ I

0 3 c\i 1

0 8 cu 1

o

i i

m i

§ i o s

o o o o o o

O O O I ?

31

experimental conditions. The reduction potentials of bipyridines are collected in Table

3.1.

3.1.1.2 Electrochemistry of N'-Methyl-2,4'-bipyridine

The cyclic voltammogram of the mono-quaternized 2,4'-bipyridine, N'-methyl-

2,4'-bipyridinium, in Fig.3.3, also shows two accessible reductions, and in contrast to

2,4'-bipyridine, both of them are chemically reversible. The doubly reduced species is

stabilized in this ionic case. It seems that the positive charge introduced by

quaternization slows down the protonation process which happens on the doubly

reduced bipyridines. The reduction of quaternized bipyridines is easier than that of the

parents, a result which can obviously be attributed to their positive charge. The

potentials of both first and second reductions shift about one volt positively on

quaternization of 2,4'-bipyridinium, as previously reported for the 4,4'-analog.29

3.1.1.3 Electrochemistry of 4,4'-Diphenyl-2,2'-Bipyridine

Fig 3.4. shows the cyclic voltammogram of 4,4'-diphenyl-2,2'-bipyridine. It

have two accessible reductions in DMF. As in the other bipyridines, the first

reduction is reversible as told by the current ratio of cathodic and anodic waves being

close to unity. The plot of its cathodic current against the square root of the scan rate

is a straight line over the range of scan rate from 0.1 to 4 V/s, as shown in the Fig

3.5, telling that the electrode process controlled by diffusion.40 The potential of this

reduction is at -2.42 V, slightly less negative than that of 2,2'-bipyridine. This may

result either from an inductive effect of the electron-withdrawing phenyl rings, or from

derealization. The second reduction is chemically irreversible, as in the other

32

Table 3.1 Cyclic Voltammetry Data for

Bipyridines"

Compounds E°/-i E-i/-2

2,4'-bipyridine -2.50/122" -3.21/irr°

N'-methyl-2,4'-bipyridine -1.52/92 -2.32/79

4,4'-diphenyl-2,2'-bipyridine -2.42/93 Not observed

2,2-bipyridine29 -2.56/74 -3.20/irr

4,4'-bipyridine29 -2.40/69 -2.88/irr

aData taken from cyclic voltammetry at 500 mV/s; measurements taken in V vs.

oxidation potential of ferrocene as internal standard, in dry DMF, at room temperature

with 0.1 M TBAPF6 as supporting electrolyte.

b ^(Epa+Epc)(V)/(Epa-Epc)(mV)

c Chemically irreversible at 4 V/s

33

o to

S UJ

(Vn) i

Fig.3.3 Cyclic Voltammogram of N'-methyl-2,4'-bipyridine in DMF (supporting

electrolyte 0.1 M TBAPF6, scan rate 200 mV/s, at room temperature)

34

o o r*N.

o o o o o o

o o o o o o

o o o o o o

o o o o o o

o © o

to in m cu

(Vn)

VI

I

o 1

cu 1

o 8

O o

ai i

o 8 > C\l I

0 s 01 t

o 0 rv oj 1

o s

35

cn GO co in

Fig.3.5 The plot of cathodic current of 4,4'-diphen-2,2'-bipy against the square root of

scan rate

36

bipyridines.

3.1.2 Electrochemistry of Platinum(II) Complexes

In general, the platinum(II) complexes investigated here show two or three

single electron reductions but no accessible oxidation under our experimental

conditions. Most of the reductions are chemically reversible as shown by their cyclic

voltammograms (potentials summarized in Table 3.2), but the separation of forward

and return waves is larger in most cases than the ideal value of 59 mV; in other

words, they are not completely electrochemically reversible. According to the

localized molecular orbital model, the reductions of these platinum(II) complexes can

be classified as ligand-based or metal-based.

3.1.2.1 Ligand-based Reductions of Platinum(II) Complexes

Bipyridines are well known for their low lying % orbitals. They usually serve

as K acceptor ligands to form complexes with a wide variety of metals. Most

complexes of this kind have a bipyridine-based LUMO and their first reduction is

bipyridine ligand-based.30"32 All the complexes studied here have one or more

bipyridine ligands. It is to be expected that their first reductions will be ligand-based.

[Pt(bipy)Cl2]

[Pt(bipy)CL>] has two accessible reductions in the solvent window of DMF as

shown in its cyclic voltammogram in Fig.3.6 and Fig.3.7. The first reduction is

chemically reversible, but the separation of the forward and return wave is 90 mV,

showing some electrochemical irreversibility. The potential of the first reduction is

-1.63 V, which is 0.93 V less negative than that of free 2,2'-bipyridine29. It is typical,

37

Table 3.2 Cyclic Voltammetry Data for

Platinum(II) Complexes11

E%' E-iw E-2/-3

[Pt(bipy)ClJ -1.63/90" -2.39/irf

[Pt(bipy)(4-Ncpy)Cl]+ -1.47/90 -1.69/100 -2.29/100

[Pt(ph2-bipy)ClJ -1.59/68 -2.18/79

[Pt(Me2-bipy)Cl2] -1.64/68 -2.38/irf

[Pt(2,4'-bipyOct-H)ClJ -1.54/90 -2.23/90

[Pt(2,4'-bipyOct)Cl3] -1.45/90" -2.04/90

[Pt(2,4'-bipyMe-H)(bipy)]2+ -1.22/80 -1.57/70 -1.96/80

[Pt(2,4'-bipyMe-H)pyJ2+ -1.44/60 -2.09/130

[Pt(DCMB)Cl2] -1.16/76 -1.81/100

[Pt(Mebipy-H)bipy]2+22 -1.46/64 -1.69/62

[Pt(Mebipy-H)pyJ2+ 22 -1.58/64 -2.12/75

"Data taken from cyclic voltammetry at 500 mV/s; measurements taken in V vs.

oxidation potential of ferrocene as internal standard, in dry DMF, at room temperature

with 0.1 M TBAPF6 as supporting electrolyte. b Vi(Epa+Epc)(V)/(Epa-Epc)(mV) 0 Chemically irreversible at 4 V/s

38

o o cu

o o CXI

«

0 1

o o to * 0 1

o o o

I ^ >

o o o o o o o o o o in in in in tn in cn OJ

o o in

o o in •

0 1

o o

o o CD

O c8 CXI

o o CO

in

UJ

(vn) i

Fig.3.6 Cyclic Voltammogram of [Pt(bipy)ClJ in DMF (supporting electrolyte 0.1 M

TBAPF6, scan rate 0.2 V/s at room temperature)

39

s O CVJ

(V«) I

Fig.3.7 Cyclic Voltammogram of [Pt(bipy)ClJ in DMF (supporting electrolyte 0.1 M

TBAPF6, scan rate 2 V/s at room temperature)

40

(vn) i

Fig.3.8 Cyclic Voltammogram of [Pt(bipy)(4-NCpy)Cl)]+ in DMF with supporting

electrolyte, 0.1 M TBAPF6, scan rate 2 V/s, at room temperature

41

when bipyridine is coordinated with a metal ion, that its reduction potential shifts

positively. This is due to the stabilization of rc(7) orbital of bipyridine by the

positively charged metal ion. The same thing happens when bipyridines are

quaternized, as discussed above.29 Klonger, Huffman and Kochi reported the first

reduction of this complex (by cyclic voltammetry in acetonitrile) in 1982. They

described the reduction product as a Pt(I) species, but this assignment seems to have

been made in a formal sense only. The reduction potential of coordinated bipyridine

is also affected by the ligands coordinated at other sites of the same metal center, due

to the change of charge distributions. In the case of [Pt(bipy)(py)J2+,22 the first

reduction, which is also bipyridine-based, happens at -1.38 V, 250 mV less negative

than the dichloro case, because this is a reduction on a dicationic species. This may

be attributed to the positive charge and/or -the change from a n donor ligand into a jc

acceptor ligand at the two other coordination sites, which stabilizes not only the

LUMO of the metal but also the LUMO of bipyridine; in the case of [Pt(bipy)ClJ, the

n(7) orbital of bipyridine is destabilized by the negatively charged chloride through L

—> M —> L' interaction.

[Pt(bipy)(4-NCpy)Cl]+

The cyclic voltammogram of [Pt(bipy)(4-NCpy)Cl]+ shows, as in Fig.3.8, three

reduction waves. The first one, at -1.47 V, is again a bipyridine-based reduction. The

potential of this reduction is about 160 mV less negative than that of [Pt(bipy)ClJ,

due to charge and/or the jc acceptor effect of 4-cyanopyridines as against the K donor

effect of chlorides. The second reduction at -1.69 V can be assigned as 4-

42

o o o o o o OJ CD

CVJ

1 1 I i i

o o o o o o o o o o o CO cu CM CO

•1 o o o o 1 *

(vn) I

Fig.3.9 Cyclic Voltammograms of [Pt(ph2-bipy)Cl2J in DMF with supporting

electrolyte, 0.1 M TBAPF6; scan rate 0.2 V/s; at room temperature

43

o o

o o o

o o

o o 03

#

0 1

o o ^ OJ

7 u.

© o CO

o o 0 OJ 1

CXI I

o o 00

o o cu

o o 00

o § O o

o o o

44

o m

C\J cu CU *"• «r1 O I

(vn) I

F ig3 . l l Cyclic Voltammograms of Pt(2,4'-bipyOct-H)Cl2 in DMF with supporting

electrolyte, 0.1 M TBAPF6; scan rate 200 mV/s; at room temperature

45

o o *

46

cyanopyridine-based.

[Pt(ph2-bipy)ClJ and [Pt(Me2-bipy)Cl2]

As shown in Fig.3.9 and Fig.3.10, the first reduction of [Pt(ph2-bipy)ClJ is at

-1.59 V and that of [Pt(Me2-bipy)Cl2] is at -1.64 V. They are both bipyridine-based.

This potential is almost the same for [Pt(bipy)ClJ and [Pt(Me2-bipy)Cy, but shifts

positively for [Pt(ph2-bipy)Cy due to the derealization or inductive effect of the

electron-withdrawing phenyl rings.

Pt(2,4'-bipyOct-H)Cl2 and [Pt(2,4'-bipyOct)Cl3]

For Pt(2,4'-bipyOct-H)Cl2, the first reduction, as shown in Fig.3.11, is at -1.54

V, very close to that of N'-methyl-2,4'-bipyridinium. We ascribe this to two opposite

effects. Coordination with platinum(II) will make the reduction of bipyridine easier,

but the replacement of a carbon-hydrogen bond by a carbon-platinum bond creates a

formal negative charge on that carbon atom, making the reduction more difficult.

Unlike Pt(2,4'-bipyOct-H)Cl2, [Pt(2,4'-bipyOct)Cl3] has only its nitrogen site

coordinating with platinum(II), without forming a carbon-platinum bond, and is a

zwitterionic overall neutral complex. Its first ligand-based reduction is at -1.45 V, as

shown by its cyclic voltammogram in DMF (Fig.3.12), about 90 mV less negative than

Pt(2,4'-bipyOct-H)Cl2.

[Pt(2,4' -bipyMe-H) (bipy)]2+

In its cyclic voltammogram (Fig.3.13), [Pt(2,4'-bipyMe-H)(bipy)]2+ shows three

reduction waves, of which the first two are based on the two separate ligands. Since

[Pt(2,4'-bipyOct-H)Cl2] is more readily reduced than [Pt(bipy)Cy, the first reduction

47

of this complex can be attributed to the 2,4'-bipyMe-H ligand and the second to the

bipyridine. The reduction potential of 2,2'-bipyridine is at -1.57 V here, which is less

negative than that of [Pt(bipy)ClJ due to the overall charge on the complex and the it-

acceptor effect of r-Methyl-2,4'-bipyridine-3-ylium.

[Pt(2,4'-bipyMe-H)py2]2+

In [Pt(2,4'-bipyMe-H)py2]2+ (Fig.3.14), the first reduction at -1.44 V, on 2,4'-

bipyMe-H, is 220 mV more negative than in [Pt(2,4'-bipyMe-H)(bipy)]2+, due to the

weaker Tt-accepting ability of pyridine compared with bipyridine. The first reduction

of [Pt(bipy)pyJ, being bipyridine-based, is at -1.38 V, which is less negative than that

of the 2,4'-bipyMe-H analog by 60 mV. Comparing at the first reductions of [Pt(2,4'-

bipyMe-H)Cy and [Pt(bipy)ClJ, one finds that the complex of 2,4'bipyMe-H is more

easily reduced. This contrast can be understood by considering that chlorine is a

negatively charged % donor, but pyridine is a neutral % acceptor. In the chlorine case,

the metal center carries less positive charge, but will still interact more strongly with

2,4'-bipyMe-H than with bipyridine. Pyridine, as a % acceptor, will increase the metal

center's positive charge, strengthening the interaction between the metal center and

either bipyridine or 2,4'-bipy-Me-H.

[Pt(Terpy)Cl]+

[Pt(Terpy)Cl]+ has three accessible reductions. Its cyclic voltammogram and

that of free terpyridine are presented in Fig.3.15 and Fig.3.16. The first two

reductions of the complex are terpyridine-based, and are chemically reversible. Free

terpyridine shows a chemically reversible reduction at -2.52 V, and the second

48

o

s o

s o

8 o

s? o §

o

s o

s o o i n

O €VJ

8 ' I D v c o OJ *•1 o o * * CM

m i

Fig.3.13 Cyclic Voltammogram of [Pt(2,4'-bipyMe-H)(bipy)]2+ in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature, scan rate 2 V/s

49

in (xj £

UJ

(V«) I

Fig.3.14 Cyclic Voltammogram of [Pt(2,4'-bipyMe-H)(py)2]2+ in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature, scan rate 0.2 V/s

50

o o o o o o o o o o c u i n i n i n i n i n i n i n i n i n i n i e u r ^ CM CVJ r ^ c u CU r ^ CU

c n OJ r u

51

o 0

o 1

o o GO

»

0 1

o o cu

o § >

UJ

o o 0 a] 1

o o

cu I

0 co cvi 1

CO ai

CVn) i

Fig.3J6 Cyclic Voltammogram of 2,2':6",2,,-Terpyridine in DMF with supporting

electrolyte 0.1 M TBAPF6 at room temperature

52

o o CXI

o o CO 0 1

o o o

o ©

tu

o §

c3 eg i

to © £ s

fv O 8

o ID % 8 8 o in cu o m rv o ru in i cu

CO CM OJ

53

reduction with cathodic wave potential at -2.98 V, for which an anodic wave could

not be detected even with a scan rate of 4 V/s. As in other cases, the reversibility of

the second reduction of terpyridine is improved on coordination to the metal ion. The

first reduction of [Pt(terpy)Cl]+ is at -1.09 V. The first reduction potential of free

terpyridine, -2.56 V, is close to the value of 2,2'-bipyridine, -2.52 V. But, perhaps

because there are three nitrogens coordinating with platinum(II) ion, constraining the

ligand to be planar, the positive shift of reduction potentials of terpyridine due to

coordination is larger than in the other cases discussed here.

[Pt(DCMB)Cy

[Pt(DCMB)ClJ has its first reduction at -1.16 V, which is DCMB ligand-based.

The cyclic voltammogram of [Pt(DCMB)ClJ is presented in Fig.3.17. It is not

surprising that the electron-withdrawing carboxymethyl group greatly lowers the

LUMO of the bipyridine.

3.1.2.2 Platinum(13)-Based Reductions

Platinum(II) complexes can undergo metal-based reduction, forming

platinum(I).29 In the complexes investigated here, most were found to have their

metal-based reduction located at around 2.0 V, as expected by analogy29 with the

earlier work. The data are collected in Table 3.2. The metal-centered reduction of

[Pt(bipy)ClJ shows some chemical irreversibility. In its cyclic voltammogram, at the

scan rate of 0.5 V/s, there is only an anodic wave centered at -2.39 V, but no

corresponding cathodic wave. The return wave for the second reduction can be

resolved at a scan rate of 2 V/s, as shown in Fig.3.7. The potential of the second

54

reduction for this complex is quite negative, which is mainly due to the % donor effect

of the dichlorine ligands. Or in other words, this is a reduction being carried out at a

complex with three negatively charged ligands (one reduced bipyridine and two chlo-

rides). In contrast, the platinum(II) centered reduction of [Pt(DCMB)ClJ is reversible

at -1.81 V, about 550 mV less negative than that of [Pt(bipy)ClJ. This is because

3,3'-dicarboxymethyl-2,2'-bipyridine is presumably a better n acceptor and/or poorer

a-donor than bipyridine, its LUMO being about 500 mV lower than that of bipyridine,

due to the electron withdrawing effect of the carboxymethyl group discussed earlier.

The metal centered reduction of [Pt(2,4'-bipyOct-H)ClJ is 160 mV less negative than

in [Pt(bipy)ClJ for the same reason; the lower LUMO of 2,4'-bipyOct-H makes it a

better it acceptor than bipyridine. The second, metal-based reductions of [Pt(2,4'-

bipyMe-H)pyJ+ and [Pt(bipy)pyJ2+ are almost at the same potential (-2.09 V and

-2.07 V22), even though their first, ligand-based reductions show that [Pt(2,4'-bipyMe-

H)py2] is more easily reduced (-1.44 V and -1.38 V22). The metal centered reduction

of [Pt(bipy)(4-CNpy)Cl]+ appears as the third wave in its cyclic voltammogram at

-2.29 V, which is much more negative than in other similar complexes. As mentioned

above, its first reduction is bipyridine-based and the second reduction is 4-cyano-

pyridine-based. So in this case the platinum(II) ion being reduced is coordinated to

three negative charged ligands, which causes a negative shift of its reduction potential.

3.2 Spectroelectrochemistry

3.2.1 2,4'-Bipyridines

The electronic absorption spectral data and proposed assignment for bipyridines

55

are collected in Table 3.3.

As shown in Fig.3.19, in DMF, the electronic absorption spectrum of neutral

2,4'-bipyridine shows only one strong band, at 273 nm, which is assigned to the TC(6)

to TC(7) transition. Bands with higher energy are not observable here due to strong

absorption by the solvent. The singly reduced 2,4'-bipyridine anion radical generated

inside the OTTLE cell gives a spectrum having the same features as that of 2,2'-

bipyridine.29 The strongest band at 388 nm is the transition rc(6) to k ( 7 ) . This band

shifts from 273 nm in the parent bipyridine to lower energy in the reduced radical

anion. This kind of shift, which is common29 in the bipyridines, is due to the added

electron on the %(7) which increases the bond order between the two pyridine rings;

Jt(7) is an antibonding orbital for the molecule as a whole but bonding between the

two rings36 (see Fig. 3.18). The bands spreading from 550 nm to 800 nm are assigned

to the transition from rc(7) to 7t( 10), and bands at 970 nm and 1128 nm to jc(7) -»

3i(8,9). Both these transition bands have clear vibrational coupling structure with

separation of 1,400 cm'1. The spectra from quaternized r-methyl-2,4'-bipyridinium in

the same experimental conditions are similar to those of 2,4'-bipyridine, as in Fig.3.20.

The 7t(6) to jc(7) transition is at 291 nm in the parent, moved to lower energy due to

the positive charge, and at 369 nm in the singly reduced species. The k( 7) to 7t(10)

transition appears at 523 nm, and n{7) to Jt(8,9) in the 1000 nm region.

3.2.2 Spectroscopy and Spectroelectrochemistry of Square Planar Platinum(II)

Complexes with 2,4'-Bipyridines as Ligands

The electronic spectra of these platinum(II) bipyridine complexes can be

56

-TT 8 u

00 , , u

4u

Fig. 3.18 7C-orbital diagram for biphenyl (After Song, J-I. PhD Thesis, University of

Glasgow, 1989)

57

2.40 r

800 1000 1200

Wavelength (nm)

1400 1600

Fig.3.19 Electronic absorption spectra of 2,4'-bipyridine and its one electron reduction

product in DMF (c = 6.6 x 10^ M) with 0.1 M TBAPF6; parent (dashed line) and

singly reduced form (solid lines)

58

2.20

8 1.32

1000 1200 Zl

1400

Wavelength (nm)

Fig.3.20 Electronic absorption spectra of N'-methyl-2,4 -bipyridmium and its o

, • n u c fc - 6 4 x lO^M) with 0.1 M TBAPF6; parent electron reduction product m DMr (c

(solid line) and singly reduced form (dashed line)

59

u o c CS X! h< O 5/3 £> <

2.80

2.236

-0.02 260 400 600

• • I

800 1000 1200 Wavelength (nm)

Fig.3.21 Electronic absorption spectra of [Pt(2,4'-bipyOctyl)Cl3j and its one electron

reduction product in DMF (c = 7.8 x 10"4 M) with 0.1 M TBAPF6 (Solid line -

parent; Dashed line ~ singly reduced)

60

Table 3.3. Spectroscopic Data and Proposed Assignments

for Platinum Complexes and Related Species in DMFa

Comp. JC(6) TT(7) Tt(7) JC(10) rt(7) JI(8,9) MLCT Other(see text)

I" 273(36.6)(20.I)

I 388(25.8)(36.7) 580(17.2)(11.6) 970(10.3X0.54) 1128(8.9X0.54)

II 290(34.5X21.2)

n 368(27.1)(33.4) 480(20.8)sh 524(19.1)(7.35)

i n 300

m 347(28.8)(17.9) 375(26.6)(17.5)

665(15.0X8.7)

IV 313(31.9X7.9) 311(32.1X6.7)

278(36.0)(18.9) 388(25.7)(8.9) 363(27.5)sh

IV 359(27.9)(10.7) 463(21.6X5.1) 913(10.1X1.5) 424(23.6X5.3)

V 311(32.2)(13.9) 323(31.0X16.6)

- 333(30.0)sh

V 273(36.6)

VI 338(29.6)(12.9) 400(25.0X7.8) 297(33.7)(47.5)

VI 367(27.2)(34.9) 430(23.3X8.5) 1117(8.9X4.8) 1279(7.8)(4.4)

524(19.1)(5.0) 562(17.7X5.7)

VII 322(31.1)(7.7) 311(32.1)(6.7)

384(26.0)(3.39) 363(27.5)sh 280(35.7X17.7)

VII 358(27.9)(5.3) 474(21.1)(2.4) 632(15.9)(0.35) 700(14.3X0.34)

510(19.6X2.8) 418(23.9)(2:.4)

61

Table 3.3.(Cont.) Spectroscopic Data and Proposed Assignments

for Platinum Complexes and Related Species iin DMP

Comp. re(6) -> jt(7) Jt(7) TC(10) rc(7) (8,9) MLCT Other(see text)

VlIIb 295(33.9X9.3) 268(37.3X11.4)* 372(26.9)sh 432(23.1)(1.2) 451(22.2)(1.2)

343(29.2)sh

v i n 319(31.3)(5.4) 473(21.1X2.7) 887(11.3X0.35) 375(26.7)(2.2)' 260(38.5) 538(18.6X2.2) 580(17.2)sh 635(15.7X0.89) 696(14.4)(0.71)

VIII2 319(31.3X5.4) 393(25.4)(4.3)

XIII 373(26.8) 436(22.9)

Xffl 450(22.2) 900(11.1)

IX

IX 371(26.9X25.2) 525(19.0X8.2)

X 312(32.1X20.2) 373(26.8X7.6)

XI 305(32.8X9.1) 316(31.6)(9.2)

359(27.8X3.6)

bipy 281(35.6)

bipy" 397(25.2)(19.1) 582(17.2)(6.1) 882(11.3X2.6)

[Pt(bipy)pyJ2+ 22 306(32.7)(18.5) 245(40.8)(10.4)

[Pt(bipy)py2]+ 22 348(28.7X8.4) 492(20.3)(6.8) 1090(9.2X3.3) 400(25.0)(7.2)

f Pt(bipy)(Me2N-py)Ji+ a 314(31.8)(17.8) 250(40.0)(9.2)

[Pt(bipy)(en)]2+ 22 321(31.2X18.4) 248(40.3)(9.0)

62

Table 3.3, Footnotes and Key

aData presented as wavelength(A,)/nm(wavenumber(v)/103 cm^Xe/lO3 M"1 cm"1)

b I, 2,4'-bipyridine, II, N'-methyl-2,4'-bipyridinium,

III, 4'-diphenyl-2,2'-bipyridine IV, Pt(bipy)Cl2

V, [Pt(bipy)(4-NCpy)Cir VI, Pt(ph2-bipy)Cl2

Vn, Pt(Me2-bipy)Cl2 Vffl, Pt(2,4'-bipyOct-H)Cl2

IX, [Pt(2,4'-bipyOct)Cl3] X, [Pt(2,4'-bipyMe-H)(bipy)]2+

XI, [Pt(2,4'-bipyMe-H)(py)2]+ Xn, [Pt(terpy)Cl]+

XIH, Pt(DCMB)Cl2

bipy, 2,2'-bipyridine

4-NCpy = 4-cyanopyridine;

Mej-bipy = 4,4'-dimethyl-2,2'-bipyridine;

ph2-bipy = 4,4'-diphenyl-2,2'-bipyridine;

2,4'-bipyOct-H = N'-octyl-2,4'-bipyridin-3'-ylium;

2,4'-bipyOct = N'-Octyl-2,4'-bipyridinium;

2,4'-bipyMe-H = N'-Methyl-2,4'-bipyridin-3'-ylium;

terpy = 2,2':6',2"-terpyridine;

DCMB = 3,3'-dicarboxymethyl-2,2'-bipyridine

MejN-py = 4-(dimethylamino)pyridine

en = 1,2-diaminoethane

63

assigned with the help of those the free ligands, as summarized in Table 3.2 and 3.3.

Pt(2,4-bipyOctyl)Cl3:

The electronic absorption spectra of [Pt(2,4'-bipyOctyl)Cl3] are presented in

Fig.3.21. The spectrum of the parent is simple. There are bands at high energy that

overlap with strong absorption background of solvent around 280 nm. These bands may

be ligand-based 71(6) to rc(7) and/or MLCT, and also some weak MLCT bands in the 400

nm region. The spectrum of the singly reduced species here is similar to those of

bipyridine anion radicals. There is a sharp strong band at 371 nm, which is the typical

rc(6) to Tt(7) transition band of bipyridine anion radicals, and a band at 525 nm which is

the ti(7) to 7i(10) transition of reduced bipyridinium. All these bands are located at

almost the same position as in reduced 1 '-methyl-2,4'-bipyridinium. These spectra clearly

imply that the first reduction of this complex is ligand-based, so that the added electron

is mainly localized at bipyridine part of complex, as is usual29 in such complexes.

Another band around 700 nm can be assigned as a ligand-based transition from 7t(7) to

7t(8,9). This band is, as usual, extremely weak, due to the cancellation of local transition

dipole moments.

[Pt(2,4'-bipyOctyl-H)Cl2]

Compared with Pt(2,4'-bipyOctyl)Cl3, the spectrum of this compound is much

more complicated and informative as shown in Fig.3.22. The reason is that in the first

case, the platinum ion is only bonded at one end of the bipyridine ligand, so there is only

one metal-ligand o bond but no 7t interaction between them. In the second case, the

situation is totally different.

In the parent spectrum, there are two well separated bands at 268 nm and 295 nm.

64

v g 1.176 x> 0

1 0.970

0.764 -

0.362 -

0.146 -

600 800 1000

Wavelength (nm)

1300

Fig.3.22 Electronic absorption spectra of Pt(2,4'-bipyOctyl-H)CI2 and its one electron

reduction product in DMF (c = 5.7 x 10"4 M) with 0.1 M TBAPF6 (Solid line -

parent; Dashed line — singly reduced).

65

The intraligand rc(6) —» it(7) transitions of unreduced free 2,4'-bipyridine and 1 '-methyl-

2,4'-bipyridinium are located at 273 nm and 291 nm respectively. According to Hanasaki

and Nagakura's work, coordination with a positively charged metal ion will cause a red

shift of this transition by several thousand wavenumbers. Therefore, the band at 295 nm

can be assigned as the intraligand Jt(6) to 71(7) transition band. The bands centered at 440

nm are from metal center to ligand n(7) charge transfer with vibrational coupling.33 The

band at 268 nm can be assigned as another metal to ligand charge transfer band, from the

HOMO of platinum(H) ion to the TC(8) orbital of the ligand. The band appearing as

shoulder at 372 nm with energy about four thousand wavenumbers higher than that of the

440 nm bands, is also a MLCT band, but it is from the next HOMO's of platinum(II)

ion center, dz2, to the JI(7) orbital of bipyridinium.36 The origin of the shoulder band at

343 nm is not very clear; it may result frotti the ligand to metal charger transfer.

The spectrum of the singly reduced form of this complex strongly suggests that

this is a ligand-based reduction, the added electron being localized on the bipyridine

ligand. There is a band at 319 nm in the spectrum of parent species, which is the Jt(6)

to jc(7) transition band of reduced ligand. This band in the reduced free ligand is at 388

nm. According to published results,22,29 the blue shift upon coordination is about 5,500

cm"1, which is consistent with this assignment. The ligand-based it(7) to Jt(10) band,

which also shifts to higher energy when bipyridines coordinated with platinum(II) ion, is

located at 473 nm. The broad bands from 538 to 700 nm can not be the MLCT bands,

because the MLCT bands should move to a higher energy due to the ligands-based

reduction, but may be assigned as a set of LMCT bands. The band at 375 nm can be

66

assigned as a MLCT band. The weak bands spreading in the region of 800 to 900 nm

are from the transition of %(1) to Jt(8), as usual for bipyridine anion radicals. The energy

and strength of all these bands are closely related to the spectrum of the reduced free

ligand, so that the first reduction is undoubtedly a ligand-based reduction, as in most

platinum bipyridine complexes. The band at 260 nm can presumably be assigned as an

LMCT band, from rc(7) of reduced ligand to the platinum(II) ion center.

[Pt(bipy)ClJ

This complex has been known for a long time.1'34 Due to its importance as an

antitumor agent,21 its properties have been broadly investigated. Gidney reported its UV/-

VIS spectra in different solvents in 1973.38,39 In 1981, Agarwala assigned its spectrum

with support from magnetic circular dichroism, and NMR studies.33

In DMF, the electronic absorption spectra of this compound, shown in Fig.3.23,

has four bands in the UV/VIS region. As published by Agarwala, the bands at 313 nm

and 325 nm can be assigned as 2,2'-bipyridine-based it to n* transitions rather than d-d

or any metal center related charge transfer band. This argument can be supported by their

extinction coefficient values (e = 7.9 x 103 and 90 x 103) which are far too high to be d-d

band, and also by the fact that they appear at almost the same position in other mixed-

ligand platinum(II) 2,2'-bipyridine complexes, such as [Pt(bipy)(4CNpy)Cl]+. The energy

difference between the two bands is about 1200 cm"1, resulting from vibrational coupling

of C — C stretching between the two pyridine rings within the same electronic transition.

The bands at 278 nm and 388 nm are MLCT bands, from platinum(II) to bipyridine-based

7c(7) and ic(8), respectively. In the spectrum of [Pt(bipy)(4-CNpy)Cl]+, where one of the

67

4.50

< 2.25

1

•—A-

800 1000

Wavelength (nm)

1200 1400

Fig.3.23 Electronic absorption spectra of [Pt(bipy)ClJ and its one electron reduction

product in DMF (c . 2.3 x 10" M) with 0.1 M TBAPF, (Solid line - parent; Dashed

line -- singly reduced).

68

chlorides which are 7C donor ligands is replaced by 4-CNpy, a kind of n acceptor ligand,

these MLCT bands shift to higher energy as the result of lowering the HOMO of the

metal ion. From the experimental data in Table 3.3, one can see that the energy order

of Pt(II) —> 7C(7) MLCT bands of Pt(II)BipyL2 complexes is as follow:

CI < en < MejNpy < py < 4CNpy

which is roughly consistent with the order of it acceptor ability.

In the spectrum of this singly reduced species, the sharp band located at 359 nm

is from the jc(6) —» jr(7) transition of the reduced bipyridine ligand, and the well separated

three bands at 450, 463 and 498 nm are from the k(1) to Jt(10) transition. The spacing

between 463 and 498 nm bands is 1500 cm"1, and is of vibrational origin. The band at

424 nm is not easy to understand; it may come from metal to ligand charge transfer. The

broad band centered at 913 nm is a typical rc(7) to rc(8) of bipyridine anion radicals.

Again, the spectrum of this singly reduced complex clearly shows localized bipyridine-

based reduction.

[Pt(bipy)(4-CNpy)Cl]+

The electronic absorption spectra of this compound, as shown in Fig.3.24, is

simple. There are only two clearly separated bands located at 311 and 323 nm, which

are the bipyridine-based tc(6) to Jt(7) transition band with vibrational coupling. Due to

the tc accepting effect of 4-CNpy, the HOMO of platinum is further stabilized so that the

MLCT band shifts to higher energy, as a shoulder overlapping with the intraligand band

at 333 nm.

The spectrum of the singly reduced form of this complex is relatively featureless.

69

There is a band at 273 nm which possibly is the metal to ligand-based Jt(8) charge

transfer band. The bands around 320 nm are from the residue of unreduced parent

compound. There are several bands spreading from 340 to 500 nm, which should include

MLCT and intraligand n(l) —> tc(10) bands. For some unknown reason, all these bands

are very weak and further assignment is difficult.

[PtDCMBClJ

Fig.3.25 shows the electronic absorption spectra of PtDCMBCl2 in DMF. The

band at 436 nm originates from metal dxy to ligand rc(7) charger transfer and the shoulder

band at 373 nm is another MLCT band which is from dz2 or doubly degenerated to

Jt(7). The separation of the two bands is about 3900 cm'1 which corresponds to the

energy gap between the two orbitals.

In the spectrum of the singly reduced form of this complex, there is a Jt(7) to

7t(8,9) transition, centered at 900 nm, which is weak and broad. The sharp band at 417

nm is the metal to bipyridine anion radical charge transfer band. The ligand-based k(7)

to 7t(10) band is at 450 nm.

70

Wavelength (nm)

UC absorption spectra of [ P U b i p y K ^ P ^ 1 1 * a n d °"e e l e C t r°"

reduction product in DMF (c = 1 -2 x

and singly reduced form (dashed toe)

71

5.0

4.5

4.0

S 3.0

J. J.

600 800 1000

Wavelength (nm)

1300

Fig.3.25 Electronic absorption spectra of [PtDCMBClJ and its one electron reduction

product in DMF (c = 2.3 x 10"3M) with 0.1 M TBAPF6, parent (solid line) and singly

reduced (all dashed lines) forms

REFERENCES

(1) Morgan, G. T. ; Burstall, F. H. J. Chem. Soc. 1934, 965.

(2) (a)Wimmer, S.; Wimmer, F. L. J. Chem. Soc., Dalton Trans. 1994,

879. (b)Morgan, G. T.; Burstall, F. H. J. Chem. Soc., 1934, 965. (c) Conran,

R. C.; Rund, J. V. Inorg. Chem., 1972, 11, 129. (d) Dholakia, S.; Gillard, R.

D.; Wimmer, F. L. Polyhedron, 1985, 4, 791. (e) Howe-Grant, M.; Lippard, S.

J. Inorg. Synth. 1980, 20, 102.

(3) Castan, P.; Dahan, F.; Wimmer, S.; Wimmer, F. L. J. Chem. Soc., Dalton

Trans. 1990, 2971.

(4) Chassot, L.; Miiller, E.; von Zelewsky, A. Inorg. Chem. 1984, 23, 4249.

(5) Wimmer, S.; Castan, P.; Wimmer, F. L.; Johnson, N. P. Inorg. Chim. Acta

1988, 142, 13.

(6) Matsubayashi, G. E.; Hirao, M.; Tanaka, T. Inorg. Chim. Acta, 1988,

144, 217.

(7) Wimmer, S.; Castan, P.; Wimmer, F. L.; Johnson, N. P. J. Chem. Soc., Dalton

Trans. 1989, 403.

(8) Sandrini, D.; Maestri, M; Ciano, L.; von Zelewsky, A. Gazz. Chim. Ital.

1988,118,661.

(9) Kamath, S. S.; Uma, V.; Srivastava, T. S. Inorg. Chim. Acta 1989, 161, 49.

(10) Zuleta, J. A.; Burberry, M. S.; Eisenberg, R

Documents

37*1 COMBINED ELECTROCHEMISTRY AND .../67531/metadc279396/...Fig.2.4c Ruthenium(II) complexes which are discussed in Chapter IV 22 Fig.2.5 Platinum(II) and palladium(II) complexes